You're a Brit, so things cost more over there, I have family there so know this.
You aren't mentioning the price range you're looking at, but as an alternative, stock bikes and or frame sets like a surly troll, 26in wheels (which I have) or a surly ogre , 700 wheels, or a bridge club, 27.5in wheels, can all be an interesting option for frame choices.
i consider my troll as my expedition bike, and have traveled with it in far off countries. It can take 2.5 inch tires with the rims I have.
Just to say that there are great Taiwan options out there as you say.
And your builder supplier guy might be very legit and knowledgeable, clearly suggesting a mtb triple is a really good recommendation. I have this on my troll, same 44/32/22 and it works really well loaded up and in mountains and on not great roads where speeds are always lower overall. But you can still pedal along at 30 or 40kph with the rare tailwind or downhill.
And yes, those bars linked with that Dunham or whatever guy with koga, look very interesting. I have drops on my troll.
